The present invention relates to aliphatic polyester microfibers, films having a microfibrillated surface, and methods of making the same. Microfibers of the invention can be prepared by imparting fluid energy, typically in the form of high-pressure water jets, to a highly oriented, semicrystalline, aliphatic polyester film to liberate microfibers therefrom. Microfibrillated articles of the invention find use as tape backings, filtration media, such as face masks and water or air filters, fibrous mats, such as those used for removal of oil from water and those used as wipes, and thermal and acoustical insulation. Microfibers of the invention, when removed from the film matrix may be used in the preparation of woven or nonwoven articles and used as wipes for the removal of debris or dust from a surface. The microfibers and microfibrillated articles of the invention may be biodegradable and/or bioabsorbable, rendering them useful for wound dressings, disposable products, and geotextiles.
Polymeric fibers have been known essentially since the beginnings of commercial polymer development. The production of polymer fibers from polymer films is also well known. Typically, molten polymer is extruded through a die or small orifice in a continuous manner to form a continuous thread. The fiber can be further drawn to create an oriented filament with significant tensile strength. Fibers created by a traditional melt spinning process are generally larger than 15 microns. Smaller fiber sizes are impractical because of the high melt viscosity of the molten polymer. Fibers with a diameter less than 15 microns can be created by a melt blowing process. However, the resins used in this process are low molecular weight and viscosity rendering the resulting fibers very weak. In addition, a post spinning process such as length orientation cannot be used.
Orientation of crystalline polymeric films and fibers has been accomplished in numerous ways, including hot drawing, melt spinning, melt transformation (co)extrusion, solid state coextrusion, gel drawing, solid state rolling, die drawing, solid state drawing, and roll-trusion, among others. Each of these methods has been successful in preparing oriented, high modulus polymer fibers and films. Most solid-state processing methods have been limited to slow production rates, on the order of a few cm/min. Methods involving gel drawing can be fast, but require additional solvent-handling steps. A combination of rolling and drawing solid polymer sheets, particularly polyolefin sheets, has been described in which a polymer billet is deformed biaxially in a two-roll calender then additionally drawn in length (i.e., the machine direction). Methods that relate to other web handling equipment have been used to achieve molecular orientation, including an initial nip or calender step followed by stretching in both the machine direction or transversely to the film length.
The production of macroscopic fibers from films has been established. Liberating fibers from oriented, high-modulus polymer films, particularly from high molecular weight semicrystalline films, has been accomplished in numerous ways, including abrasion, mechanical plucking by rapidly-rotating wire wheels, and impinging water jets to slit the film. Water jets have been used extensively to cut films into flat, wide continuous longitudinal fibers for strapping or reinforcing uses.
Pennings et.al. in xe2x80x9cMechanical properties and hydrolyzability of Poly(L-lactide) Fibers Produced by a Dry-Spinning Methodxe2x80x9d, J. Appl. Polym. Sci., 29, 2829-2842 (1984) described fibers with a fibrillar structure by solution spinning using chloroform in the presence of various additives (camphor, polyurethanes) followed by hot drawing. These fibers showed good mechanical properties and improved degradability in vitro with the fibrillar structure speeding up the hydrolysis of the fiber. The inherent disadvantage of this process is the use of chlorinated solvents in the spinning process.
Microfibers with a diameter of 1 micrometer and a round cross section have also been produced by electrospinning. The electrospinning technique also suffers from the disadvantage of using a chlorinated solvent and has low production speeds.
WO 95/23250 discloses a process for preparing biodegradable fibrils from polylactide where a polymer solution is precipitated into a non-solvent. The fibrils can be dried and formed into a biodegradable nonwoven article.
U.S. Pat. No. 6,111,060 (Gruber et al.) discloses the use of melt stable polylactides to form nonwoven articles via melt blown and spunbound processes. These fibers have low orientation and have generally low tensile strength. In addition, the fibers have a round cross sectional area comparable to traditional textile fibers.
WO 9824951 discloses the production of multicomponent fibers for nonwovens comprising two different polylactides.
The present invention is directed to aliphatic polyester microfibers having an average effective diameter less than 20 microns, generally from 0.01 microns to 10 microns, and substantially rectangular in cross section, having a transverse aspect ratio (width to thickness) of from 1.5:1 to 20:1, and generally about 3:1 to 9:1. Since the microfibers are substantially rectangular, the effective diameter is a measure of the average value of the width and thickness of the microfibers. The cross-sectional area of the fibers is generally from about 0.05 to 3.0xcexc2, and typically 0.1 to 2.0xcexc2.
The rectangular cross-sectional shape advantageously provides a greater surface area (relative to fibers of the same diameter having round or square cross-section) making the microfibers (and microfibrillated films) especially useful in applications such as filtration and as reinforcing fibers in cast materials. The surface area is generally greater than about 0.25 m2/gram, typically about 0.5 to 30 m2/g. Further, due to their biodegradability and/or bioabsorbability, the microfibers of the present invention are useful in applications such as geotextiles, as suture materials and as wound dressings for skin surfaces.
The present invention is further directed toward the preparation of microfibrillated articles, i.e. highly-oriented films having a microfibrillated surface, by the steps of providing a highly oriented, voided or microvoided, aliphatic polyester film, and microfibrillating said voided film by imparting sufficient fluid energy thereto. The fluid energy may be imparted by a high pressure fluid jet or by ultrasonic agitation. As used herein, the term xe2x80x9cmicrofibrillated articlexe2x80x9d refers to an article, such as a film or sheet bearing a microfibrillated surface comprising microfibers prepared from oriented films. Optionally the microfibers may be harvested from the microfibrillated surface of the film.
The voided film may be an aliphatic polyester microvoided film, or a voided film prepared from an immiscible mixture of an aliphatic polyester and a void-initiating particle. As used herein, the term xe2x80x9cfilmxe2x80x9d shall also encompass sheets, including foamed sheets and it may also be understood that other configurations and profiles such as tubes may be provided with a microfibrillated surface with equal facility using the process of this invention. As used herein, the term xe2x80x9cvoidedxe2x80x9d shall also include xe2x80x9cmicrovoidedxe2x80x9d.
Advantageously the process of the invention is capable of high rates of production, is suitable as an industrial process and uses readily available polymers. The microfibers and microfibrillated articles of this invention, having extremely small fiber diameter and both high strength and modulus, are useful as tape backings, strapping materials, films with unique optical properties and high surface area, low density reinforcements for thermosets, impact modifiers or crack propagation prevention in matrices such as concrete, and as fibrillar forms (dental floss or nonwovens, for example). The microfibers and microfibrillated articles may be used in applications where biodegradability and or bio-absorbability are desirable. Such applications include, bandages, and wound dressings, packaging materials such as bags, tape or cartons, personal hygiene products, and geotextiles, such as those used for stabilization, protection or drainage of soils.
The process of the invention produces a fiber having a high degree of uniaxial orientation resulting in high strength, modulus, and toughness compared to prior art processes for producing microfibers. Furthermore, the process does include the use of solvents that are costly and possibly harmful. The fibers also have a unique cross sectional aspect ratio xe2x89xa71.5 and an effective diameter of less than ten micrometers, generally less than 5 micrometers.
As used herein, xe2x80x9cbiodegradablexe2x80x9d is meant to represent that the microfibers or microfibrillated articles degrade from the action of naturally occurring microorganisms such as bacteria, fungi and algae and/or natural environmental factors.
As used herein xe2x80x9cbioabsorbablexe2x80x9d means that the microfibers or microfibrillated articles may be broken down by biochemical and/or hydrolytic processes and absorbed by living tissue.